1. Field of the Invention
This invention relates generally to structures for storage of hazardous materials, and more particularly to a monolithic secondary containment vault system for isolating material storage tanks which is formed of slurry infiltrated fiber concrete.
2. Brief Description of the Prior Art
In the past, materials such as petroleum products, chemicals, and hazardous materials have been stored in large metal or fiberglass tanks which are buried underground. Most of these underground storage tanks are prone to leakage due to being subjected to the hydrostatic forces of ground water, physical stresses associated with ground movement, and the corrosive action of soil environments. These steel tanks are known to begin failing leakage tests or to begin leaking at a much greater frequency after about twelve years in operation. Great damage to the environment and personal injury often results when the leaked materials enter the soil or ground water.
The United States Environmental Protection Agency (EPA) has recently adopted new regulations for Underground Fuel Storage Tanks (UFST) in response to the growing awareness of the damage caused by releases from the UFST's. These regulations will require UFST owners to spend significant sums of money over the life of the storage tanks for monitoring, reporting, and corrective actions. Failure to comply with the EPA regulations could result in having to take the storage tank out of service, and the possibility of financial liability for property damage and personal injuries. The EPA has estimated that more than $69 billion will be spent over the next 30 years on UFST systems in leak detection, inspections, upgrading, and corrective actions.
One method to comply with the EPA regulations is to place the fuel storage tank inside a buried "secondary containment vault". The secondary containment vault is a box-like structure having an interior volume greater than the capacity of the tank it contains. Such a system provides the ability to easily monitor the tank for leakage. Should a leak occur, the secondary containment vault will completely contain the leak, preventing the fuel from entering the soil or ground water. The secondary containment vault also isolates the fuel tank from soil and hydrostatic pressures and the corrosive action of many soils. Fuel tanks which are situated in secondary containment vaults in a manner to allow physical inspection are specifically excluded from EPA and most state regulations.
Most underground secondary containment vaults currently available are fabricated using conventional reinforced and pre-stressed concrete. To meet the structural design requirements for resisting hydrostatic and soil pressures, the walls of the vaults are generally from 8 to 10 inches thick. This produces a structure which is too heavy to be transported or shipped as a single unit. As a result, most conventional secondary containment vaults are manufactured in three parts; a monolithic lower section, an upper section, and a roof slab for the upper section. The roof slab is manufactured in several panels. To develop the required structural capacity of the vault wall, and to insure a leak-free joint between the lower and upper sections, post tensioned cables are used to draw the two sections together after the components have been assembled in the excavation. Rubber gaskets and caulking are employed to make the joint leak free. Such a secondary containment vault is manufactured by SCV Corp. of San Antonio, Tex.
Another conventional precast concrete secondary containment vault is manufactured by Utility Vault Company, Inc., of Pleasanton, Calif.
The disadvantages of the conventional three-part concrete secondary containment vault are overcome by the present secondary containment system which is a monolithic vault system formed of slurry infiltrated fiber concrete having thinner walls and a gross weight significantly less than conventional reinforced and pre-stressed concrete vaults of the same size. As pointed out hereinafter, the concrete composite material is quite different from "steel fiber reinforced concrete" in both its fiber volume density and in the way it is manufactured.
There are several patents which disclose various fiber reinforced concrete structures.
U.S. Pat. No. 3,429,094 to Romualdi discloses a two-phase concrete and steel material comprising closely spaced short wire segments uniformly distributed randomly in concrete wherein the average spacing between wire segments is not greater than 0.5 inches.
Fleischer et al, U.S. Pat. No. 4,257,912 discloses a system for fixed storage of spent nuclear fuel having activated fission products contained within a metallic fuel rod housing which comprises a uniform concrete contiguously and completely surrounding the metallic housing which has metallic fibers to enhance thermal conductivity and polymers to enhance impermeability for convectively cooling the exterior surface of the concrete.
Lankard et al, U.S. Pat. No. 4,559,881 discloses a burglar resistant security vault formed of prefabricated steel fiber reinforced concrete modular panels.
Double et al, U.S. Pat. No. 4,780,141 discloses a cementious composite material containing metal fiber which particularly formulated to have high strength and a high degree of vacuum integrity at high temperatures. The composite comprises a high strength cement matrix and a filler component comprising a metal fiber having a length of about 0.05 mm. to about 5 mm. (about 0.02" to about 0.20"). The metal fiber filler is mixed with the cement matrix at a high vacuum to minimize air bubbles and then the liquid mixture (including metal fiber) is poured into the mold.
The present invention is distinguished over the prior art in general, and these patents in particular by a secondary containment structure formed of a slurry infiltrated fiber concrete composite which is used above ground or underground to enclose material storage containers and to safely contain any materials leaked from the container. The structure is a hollow configuration having a bottom wall, at least one side wall, and a removable top wall. The interior volume of the structure exceeds the volume capacity of the container which is enclosed therein. The bottom wall is sloped to facilitate drainage of any liquid leaked from the container and the top wall may have covered apertures to allow access to the container. The structure is formed of a slurry infiltrated fiber concrete composite which is produced by first placing a plurality of individual short fibers or fiber mats of organic or inorganic materials into a form to create a bed of fibers substantially filling the form and having a predetermined fiber volume density and then adding the slurry mixture into the form to completely infiltrate the spaces between the fibers. The slurry mixture includes a composition of Portland cement, fly ash, water, a high range water reducer (superplasticizer), and may also include fine grain sand, chemical admixtures, and other additives. Due to its fiber volume density and method of manufacture, the resulting secondary containment structure has thinner walls, greater strength, and a gross weight significantly less than conventional reinforced and pre-stressed concrete structures of the same size.